I've taken out the computer in my truck and I 've got just about got everything to work. The biggest problem I'm having is the brake lights. The running lights and blinkers work. I've scanned the diagram for you all to see. I've found that the purple wire coming for my switch isn't hooked up I cut it. I'm thinking that it may have went to the computer. Any Idea's on a quick fix?

You say that the parking lights and running lights work, just not the brake lights, right? If this is true, is the way you are checking just by seeing if the brake light bulb lights or are you using a test light at the harness connector? If just the bulb check is the way you are check with a test light. These trucks have a lot of problems with the rear light(s) boards. It is possible that both have failed. Also you wire diagram looks like it is for 1996 and later models.

I've taken out the computer in my truck and I 've got just about got everything to work. The biggest problem I'm having is the brake lights. The running lights and blinkers work. I've scanned the diagram for you all to see. I've found that the purple wire coming for my switch isn't hooked up I cut it. I'm thinking that it may have went to the computer. Any Idea's on a quick fix?

If you have taken the computer out of your truck, I assume you have a carb, no lock-up converter or anti-lock brakes.

Yes, you are correct that the purple wire goes to the computer. 1990 GM truck - This will be similar to my 94' Suburban. You should have a four wire brake/TCC switch under the dash. The pink/blk and purple wires feed the ECM (PCM w/ 4L60E). The pink wire is 12v ignition-on source and goes to the part of the switch that is normally closed. This feeds voltage to the ECM (or PCM) via the purple wire when the brake pedal is not depressed. The ECM (or PCM) uses this signal for TCC (and/or 4L60E) controls, cruise control and anti-lock brakes (if equipped). So, if you press the brake pedal the voltage to the ECM (PCM) is interrupted.

Removing the ECM and not having the purple wire input will not affect the brake lights on a 1990 truck; only TCC, cruise and anti-lock brakes if still functioning. If your brake lights are not working, you need to check the orange and white wires. One of these (I can't remember) is a full-time 12v battery source. It feeds the part of the switch that is normally open. So it only makes the connection when the brake pedal is depressed. I think the orange is the battery side of the switch and the white is the brake light side of the switch. Use a test light to probe and determine which is which.

I had this switch go bad and the section controlling the pink and purple wires was intermittent. I love my cruise control and this problem would randomly turn it off. It became very frequent and I also noticed the TCC lock-up going in and out at cruise. By using my Auto-X-Ray scanner and researching for about two weeks I determined I needed a new switch, which is about $15 at GM.

If your brake lights are not working, check the orange and white wires. Using a test light will easily determine if it is the switch. Don't worry about the purple wire if you are not controlling TCC, cruise and/or anti-lock brakes; but be sure to put some electrical tape or a wire nut on the cut end.

You should have (Depending on your specifics) a set of contacts for power, a set of contacts for lights, a set of contacts for Cruise control, a set of contacts for Computer..(some of these can be shared contacts)

Isolate the set of contacts that support your brake lamps, Then find the center wiper on that switch for those contacts...Re~fuse, and test for power..My guess here is in grafting the Computer harness, you cut constant power to the brake light system all together..

To test in Bits, just pull the lamp wires off the switch, and heat them up with a fused 12 volt source..the lights should come on...not blow a fuse.

Then still disconnected, heat up the power contacts on the brake light switch the fuse should remain stable, and you should be able to meter 12 volts OUT a set of contacts with the pedal depressed (or even in a bad mood...lol..) Those are the 2 you need to hook up.

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